Not sure I totally understand your question numpty, but the oxy system on pressurised aircraft is supplied from a number of oxygen cylinders located somewhere within the aircraft. I can't answer specifically for the ATR but the 747-400 ones are located in the sidewall fwd freight lining. On this aircraft the crew and pax oxygen are two seperate systems. The passenger system has a ringmain running in the PSU rail (alongside the reading light and attendant call panels overhead) This is normally unpressurised until a decompression or a manual selection from a flight deck switch activates a valve down in the freights and pressurises the lines with oxy. The dropdown doors which allow the masks out in the cabin are have a small pressure switch which react to high cabin altitude (can't remember the figure) or will drop if the pilot moves that guarded switch in the flight deck to ON.
The crew oxygen line is pressurised constantly, its there, right at the mask whenever it is needed. The -400 ones wrap around your head and can be put on in seconds. On the front of them, there is a selector switch which allows the crew to select either mixed oxygen or 100% pure (i.e. straight from the cylinder into your gob!)
Under normal operation, the oxygen has no part to play in keeping the pax alive and well. Ambient air is ducted from the compressor of the engine, passed through various precoolers and a/c packs etc. and is fed into the cabin with plenty of oxygen partial pressure.
hope that helps!